This invention relates to a drive control apparatus for controlling a motor to drive the compressor of an air conditioner, and particularly to a novel arrangement of the circuits constituting the drive control apparatus.
The motor in the conventional air conditioner used in homes and offices has a constant revolution speed depending on the frequency of the commercial power supply in the area in which the conditioner is used, and hence the cooling/heating ability is also constant. Therefore, since the cooling/heating ability cannot be directly adjusted, the compressor is controlled to be on and off, or to intermittently operate, thereby adjusting the room temperature. Thus, the efficiency of the air conditioner is low. In addition, at the start of the cooling/heating operation, cooling/heating ability is insufficient.
Recently, an inverter circuit including semiconductor switching devices has been used for the control of a motor in order that changes to revolution speed (frequency) of the motor and the voltage can be controlled, and this inverter has also been employed in the air conditioner. Use of the inverter circuit for the control of the motor in the air conditioner makes it possible to control the revolution speed of the compressor and hence to change the cooling/heating ability thereof. This leads to the reduction of energy consumption and to a more comfortable cooling/heating effect.
FIG. 1 shows an arrangement of the inverter-incorporated drive control circuit for the air conditioner. In FIG. 1, there are shown a compressor-driving power converter 1, a compressor motor 2, a compressor 3, a signal controller 4, an inverter 5, a rectifying circuit 6, a base drive circuit 7, a microcomputer 8, a sensor 9, a commercial power supply 10, a power-factor improving reactor 11, a power-factor improving capacitor 12, a current detecting resistor 13, a smoothing capacitor 14, diodes 15 to 18, circulation diodes 19 to 24, power transistors 25 to 30, a temperature detector 31 for the compressor 3, a microcomputer control circuit 32, and a DC voltage source 40.
As shown in FIG. 1, the inverter-incorporated drive control apparatus for the air conditioner has a larger number of circuit components and more kinds of components than the conventional drive control apparatus having no inverter, and the circuit construction is more complicated than the conventional one. The air conditioner for home use and office use is desired to be as small as possible, but the inverter-incorporated air conditioner is inevitably large.
The drive control apparatus of FIG. 1 will be further described in detail.
Referring to FIG. 1, the compressor driving power converter 1 includes a filter formed of the reactor 11 and the capacitor 12, the rectifying circuit 6 formed of the diodes 15 to 18, the smoothing capacitor 14, the resistor 13, the inverter 5 and the base drive circuit 7. The AC voltage from the commercial AC power supply 10 is supplied through the LC filter of the reactor 11 and the capacitor 12 to the rectifying circuit 6 and the smoothing capacitor 14, thereby being rectified and smoothed into a DC voltage. This DC voltage is applied to the inverter 5.
On the other hand, the microcomputer 8 is controlled by the microcomputer control circuit 32 so as to produce a rotation control signal (PWM) signal in response to the temperature of compressor detected by the temperature detector 31 and the ambient temperature detected by the sensor 9. The base drive circuit 7 is controlled by this control signal so as to drive the power transistors 25 to 30 of the inverter 5. The inverter 5 is thus controlled by the control signal so as to convert the DC voltage into a three-phase drive current and supply it to the motor 2. The microcomputer 8 detects the change of the load on the compressor 3 from the current flowing in the resistor 13, and controls the revolution speed of the motor 2 in accordance with this load change.
In order for the inverter 5 to generate a large drive current, the base drive circuit 7 drives the inverter 5 at a high voltage under the control of the microcomputer 8. The base drive circuit 7 is shown by one block in FIG. 1 for convenience of explanation. In practice, the base drive circuit is provided for each power transistor so that the drive signal is supplied to the base of that power transistor from that base drive circuit as illustrated. The power transistors 28, 29 and 30 of the lower arm of the inverter 5 are driven by a common base drive circuit which is supplied with a DC voltage from the DC power supply 40. The power transistors 25, 26 and 27 of the upper arm of the inverter 5 are, respectively, driven by three separate base drive circuits which are supplied with DC voltages from the DC power supply 40.
The base drive circuit 7 has a portion which treats a high voltage. Thus, the microcomputer 8 must be electrically separated from the high voltage treating portion. According to Japanese Registration of Utility Model Publication Gazette No. 63-5436, a photocoupler is used as this separate means. The base drive circuit 7 is also provided with an overcurrent protection circuit for protecting the power transistors 25 to 30 of the inverter 5.
FIG. 2 is a circuit diagram of the drive portion of an example of the base drive circuit 7 shown in FIG. 1. This example is disclosed in the above-given Gazette No. 63-5436, and used for the power transistor 25 of the inverter 5.
Referring to FIG. 2, an output pulse signal (the control signal) from the microcomputer 8(FIG. 1) is supplied through input terminals 33, 34 and through a photocoupler 35 to a pulse amplifying circuit. This pulse amplifying circuit is electrically separated from the microcomputer 8 by the photocoupler 35. The collector of the phototransistor within the photocoupler 35 is connected to the high-potential side (6 V side as illustrated) of an internal DC power supply (not shown). The emitter of the phototransistor is connected through a series circuit of resistors R4, R5 to the low-potential side (ground). The junction between the resistors R4, R5 is connected to the base of an NPN transistor Tr1. The collector of this NPN transistor Tr1 is connected to the high-potential side of the internal DC power supply through a series circuit of a resistor R6 and a forward-biased diode D1. The emitter of the NPN transistor Tr1 is connected to the low-potential side of the internal DC power supply.
The junction of the diode D1 and the collector of the transistor Tr1 is connected through a resistor R7 to the base of a first PNP transistor Tr2. The base of the transistor Tr2 is also connected through a resistor R8 to the high-potential side of the internal DC power supply. In addition, this transistor Tr2 has its emitter directly connected to the high-potential side of the internal DC power supply, and its collector connected through a resistor R9 to the base of the power transistor 25.
The junction between the diode D1 and the resistor 6 is connected through a diode D4 to the base of an NPN transistor Tr3. The collector of this transistor Tr3 is connected through a resistor R10 to the resistor R9 and to the base of the power transistor 25. The emitter of the transistor Tr3 is connected to the low-potential side of the internal DC power supply. The emitter and base of this transistor Tr3 are connected through a resistor R11.
Also, the emitter of the power transistor 25 is connected to the low-potential side of the internal DC power supply through a parallel circuit of two serially connected forward-biased diodes D2, D3 and a capacitor C.
When the output pulse signal from the microcomputer 8 is supplied through the input terminals 33, 34 to the above-mentioned drive portion, the photocoupler 35 is turned on, allowing the base current to flow to the base of the transistor Tr1 through the resistor R4, so that the transistor Tr1 is turned on. Thus, a current is flows through the resistor R6 so as to reduce the potential of the base of the transistor Tr2, so that the transistor Tr2 is turned on. Therefore, a base forward current I B1 flows to the base of the power transistor 25 through the transistor Tr2 and resistor R9, thus turning the power transistor 25 on. When the power transistor 25 becomes conductive, a current flows into the capacitor C through the emitter of the power transistor 25, charging it to the voltage corresponding to the voltage drop across the two serially connected diodes D2 and D3.
In order to speed up the charging to the capacitor C, a resistor R0 may be connected between the junction of the capacitor C and the emitter of the power transistor 25 and the high-potential side of the internal DC power supply as indicated by the broken line.
When the transistor Tr2 is in the on-state, the transistor Tr3 is turned off since its base is at the low potential. Therefore, when the output pulse signal from the microcomputer 8 is not supplied to the photocoupler 35, the photocoupler 35 is turned off, and thus the transistor Tr1 becomes nonconductive. When the transistor Tr1 is turned off, the transistor Tr2 is turned off since its base is at the high potential, so that the base current I B1 does not flow to the base of the power transistor 25. At the same time, the transistor Tr3 is turned on since its base is at the high potential. In this case, the emitter of the power transistor 25 is at a higher potential than the low-potential side of the internal DC power supply because of the charged voltage across the capacitor C. Thus, a reverse current I.sub.B2 flows for a short time to the base of the transistor Tr3 through the resistor R10 from the base of the power transistor 25. This base reverse current I.sub.B2 serves to rapidly discharge the accumulated charge on the capacitor C and the accumulated charge between the base and emitter of the power transistor 25, thus turning the power transistor 25 off.
Accordingly, the power transistor 25 has a high switching speed since it can be quickly and accurately turned on and off.
FIG. 3 is a perspective view of the construction of the drive control apparatus mentioned above.
Referring to FIG. 3, the power transistors 25 to 30, serving as chip components, are mounted on a module 36. In addition, on a circuit board 37 there are mounted the photocouplers 35 and pulse amplifying circuits of the base drive circuit 7, the overcurrent protecting circuit 39 and internal DC power supply incorporated within the base drive circuit 7, and the microcomputer 8. The drive control apparatus in the prior art is thus formed of the semiconductor devices which are mounted as discrete components on the circuit board as described above.
FIG. 4 is a block diagram of the circuit arrangement of the drive control apparatus of such construction as shown in FIG. 3. The internal DC power supply 40 generates and supplies DC voltages to the respective power transistors 25, 26 and 27, and a DC voltage common to the power transistors 28 to 30.
Furthermore, as released in the Japanese Denpa Shimbun dated Mar. 22, Heisei 2(1990), published by the Denpa Publications Inc., a three-phase inverter (hereinafter, referred to as the one-chip three-phase inverter) has been developed which uses a DC source voltage obtained by rectifying and smoothing the commercial AC voltage of 100 V, and controls the revolution speed of the motor by the dielectric-separation monolithic structure.
As patent applications concerning the one-chip inverter, there are Japanese Patent Application Laid-open Gazettes No. 3-226291 filed Jan. 31, 1990, No. 3-270677 filed Mar. 20, 1990, and No. 63-233431 filed Sep. 20, 1988, U.S. Pat. Nos. 4,890,009, 4,841,427, 5,008,586, and 5,057,721.
The dielectric-separation method will be briefly described below. In the conventional separation technique using PN junctions, a latch-up phenomenon occurs as the voltage is increased, and normally at 100 V or above, reliability cannot be assured. Thus, if the insulation between the devices can be satisfactorily performed by the dielectric separation technique, the breakdown voltage of several hundreds of volts can be assured and the commercial source voltage can also be used. The device structure of the one-chip three-phase inverter using the dielectric separation technique, as shown in FIG. 5A, employs polysilicon (Poly-Si) as the base and has the respective-phase areas partitioned to be resistant to high voltages by the dielectric separation means, or SiO.sub.2 layers so that one-phase circuit can be formed within each of the areas.
FIG. 5B is a plan view of this one-chip three-phase inverter, showing the layout of the respective devices. This monolithic semiconductor integrated circuit chip inverter includes 6 power transistors 25 to 30 which make switching operation as main devices, 6 diodes 19 to 24 which are respectively connected between the collector and emitter of the power transistors 25 to 30 so as to turn off the power transistors 25 to 30, a logic circuit 41 for generating switching control signals by which the power transistors 25 to 30 are turned on and off, the drive circuit 7 for driving the power transistors 25 to 30 to be turned on and off in response to the switching control signals, the overcurrent protection circuit 39 which detects the currents flowing in the power transistors and prevents the integrated circuit IC from being broken by the overcurrents, and the internal DC power supply 40. This one-chip three-phase inverter IC has the dimensions of 4.3 mm in length and 5.8 mm in width.
A lateral-type IGBT (Insulated Gate Bipolar Transistor) has been developed and employed for the power transistors 25 to 30 of the one-chip three-phase inverter. Thus, the occupied area can be greatly reduced as compared with that in the conventional power MOSFET. In addition, a high-speed diode which can be produced by the same process as that of the lateral-type IGBT has been developed and used for the circulation diodes 19 to 24. Thus, the reverse recovery current can be greatly decreased so that the switching loss of the power transistors 25 to 30 due to the reverse recovery current can be reduced considerably.
In addition, since the internal DC power supply 40 is incorporated in the one-chip three-phase inverter, only the external power supply may be provided for driving the power transistors 25 to 30 as the power devices. Also, since the overcurrent protection circuit 39 is incorporated in the one-chip three-phase inverter, this IC can be prevented from being broken down by the overcurrent which occurs if a short circuit should form across the load, for example. Moreover, the inverter frequency is selected to be a higher frequency of 20 kHz than the audio frequencies so that the noise of the motor can be reduced.
In the above-mentioned conventional drive control apparatus, however, as shown in FIG. 3 the electrical parts other than the module unit of inverter 36 are mounted as discrete components and as semiconductor devices on the circuit board 37, and the DC power supply 40 must generate 4 different DC voltages, that is, DC voltages to the three respective power transistors of the upper arm of the inverter and a DC voltage common to the three power transistors of the lower arm, and is therefore of considerable size. Since this large internal DC power supply is also mounted on the circuit board, the circuit board 37 is required to be considerably large, thus occupying a wide area, which inevitably makes the air conditioner large. This large size problem becomes serious in the air conditioners for smaller rooms.
In addition, the conventional drive control apparatus includes the photocoupler 35 as described above. The PWM frequency of the inverter of this drive control apparatus is now about 2 to 5 kHz. Under the use of this photocoupler 35, the power transistors of the inverter can be turned on even if the switching speed is relatively slow. However, when the PWM frequency of the inverter increases to tens of thousands of Hz as in the one-chip three-phase inverter using the dielectric separation, an expensive photocoupler capable of high speed switching becomes necessary.
The photocouplers which are used for the respective power transistors of the inverter are irregular in their switching speed, and thus the motor cannot be driven smoothly enough. Therefore, the dead time, for instance, must be increased.
Furthermore, the maximum output current of the above-mentioned one-chip three-phase inverter is about 1 ampere (A) at present, and thus the output capacity of the motor as the load is about 50 watts (W), maximum. Therefore, this inverter cannot drive such a motor as is required to produce output of over 1500 W in the compressor of the air conditioner for a room.